Such apparatus includes typically a mechanically oscillatable unit which executes mechanical oscillations in at least one direction and which is secured at a seating; an exciting/receiving unit which excites the mechanically oscillatable unit to execute mechanical oscillations and which receives the mechanical oscillations of the mechanically oscillatable unit; and a control/evaluation unit which controls the mechanical oscillations of the mechanically oscillatable unit and which, with the mechanical oscillations of the mechanically oscillatable unit, determines, i.e. ascertains, and/or monitors the process variable. The process variable can be, for example, a fill level, or, in the case of liquids as the medium, also, e.g. density or viscosity. Other process variables are, however, likewise possible.
Such an apparatus is disclosed, for example, in patent DE 39 31 453 C1. The principle of measurement of the fill level with such measuring devices is that the oscillations of a mechanically oscillatable unit, e.g. an oscillatory fork, depend on whether the unit is covered by the medium or not. The covering decreases e.g. the amplitude. Conversely, when the amplitude increases, it can be concluded that the medium has fallen below the fill level, as determined by the dimensions of the sensor and the location of the installation. Simultaneously, in the case of covering, however, the density and viscosity of the medium can also be determined. For this it is, e.g., necessary to evaluate, besides amplitude and frequency, also the phase between the exciting signal and the received (response-)signal.
The mechanically oscillatable units are usually so constructed that, as much as possible, no reaction forces and moments arise in the region of the seating (usually involving a membrane, or diaphragm) of the mechanically oscillatable unit on the measuring device. This can usually be accomplished in the case of the above-mentioned oscillatory fork by providing both oscillatory rods with equal mass moments of inertia and stiffnesses, especially thus both tines should be secured equally and as much as possible symmetrically on the membrane. If both tines oscillate with opposite sense with reference to one another, then the forces and moments exactly cancel and no reaction forces and moments act on the seating, respectively on the membrane. In this way, preventing oscillatory energy lost through the seating is prevented. Problematic, however, is the case in which the oscillatory rods are not arranged perpendicularly on the seating, but instead are slightly inclined away from, or toward, one another. The oscillatory fork is thus, quasi, spread apart or pressed together. In this case, axial forces arise parallel to the axis of symmetry of the tines. Since these forces are directed in the same direction, they do not cancel, but instead act on the seating and are consequently coupled to the environment, a fact which can lead to a loss of energy.